From US 2003/0043603 A1 a frequency converter is known, wherein an energy flow from a three-phase network is fed from secondary windings of a transformer with one or more cells per output phase. For each output phase, each identical cell has a diode input rectifier circuit, an active harmonic filter, an intermediate circuit capacitance and a three-point DC/AC converter on the output side.
Document JP 04138063 A discloses a bridge type rectifier circuit RC composed by bridge-connecting thyristors as controlled rectifiers and diodes.
Document JP 2004072846 A discloses a rectifying device, wherein a plurality of serial connection circuits are connected in parallel which include a switching arm composed of an inverse parallel connection circuit of a switching element and a diode, a serial connection circuit in which a diode is connected in series to any one of the switching elements having an inverse conduction function, and a serial connection circuit in which two diodes are connected in series.
Document U.S. Pat. No. 6,055,167 discloses a boost converter, wherein an input sinusoidal AC voltage source is input to an EMI filter and surge suppressor, wherein an output of the EMI filter is fed to input bridge rectifier consisting of two power rectifiers and two SCR's, which are gated by a phase control circuit, wherein by controlling the firing angle of the SCRs with respect to zero crossover point, the output voltage (DC OUT) is controlled.
Document U.S. Pat. No. 4,122,515 discloses a power converter with a plurality of rectifier circuits, in which two sets of rectifier circuits are employed, wherein a rectifier circuit system is comprised of a bridge circuit including a pair of thyristors and diodes, and another bridge circuit including thyristors and diodes. Thereby, the DC sides of the bridge circuits is connected in series.
Document DE 26 01 263 discloses a power converter for converting a AC voltage to a DC voltage having plural rectifier circuits.
However, the operation of the conventional frequency converter is difficult, in particular during starting up of the converter. Further, when a single cell in the frequency converter comprising several cells fails, energy supply to the load may be completely interrupted. Thereby, many disadvantages may occur, in particular for a subsea operation of a device, such as for example a pump or other equipment driven by a motor.
There may be a need for a rectifier stage for rectifying an input AC voltage to a DC voltage, a need for a converter for converting an input AC voltage to an output AC voltage and a need for an assembly of converters for converting an assembly input AC voltage to an assembly output AC voltage, wherein the above-mentioned problems are at least partially overcome or at least reduced.